The present invention relates to an active clamp DC-DC converter.
FIG. 5 shows a circuit diagram of a conventional active clamp DC-DC converter. In the DC-DC converter designated generally by 50 in FIG. 5, when the main switching device 51 (MOSFET) is turned on, an excitation current flows from the DC power source 52 through the capacitor 53 to the primary coil of the transformer 54, so that a current flows through the diode 56 due to the electromotive force of the transformer 54. The current flowing through the diode 56 is output as DC power to the load through the rectifying circuit 57 having the diodes 55, 56 and the smoothing circuit 60 having the inductor 58 and the capacitor 59. When the main switching device 51 is turned off, a current flows through the diode 55 due to the electromotive force of the inductor 58. The current flowing through the diode 55 is output through the smoothing circuit 60 to the load as DC power. In this way, the main switching device 51 is turned on and off alternately so that constant DC output voltage is obtained from the DC power source 52 through the transformer 54. Such configuration is commonly known, as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2005-245097.
In the DC-DC converter 50, when the main switching device 51 is turned from on to off, the excitation current flowing through the primary coil of the transformer 54 flows through the parasitic diode of the reset switching device 61 (MOSFET) to the capacitor 62, so that the capacitor 62 is charged and the excitation current flowing through the primary coil of the transformer 54 is decreased. When the reset switching device 61 is then turned on, the energy in the capacitor 62 is discharged to the primary coil of the transformer 54 and the excitation current is further decreased, thereby resetting the transformer 54. In this way, resetting of the transformer 54 in the DC-DC converter 50 is done by an active clamp circuit composed of the reset switching device 61 and the capacitor 62.
In the DC-DC converter 50, the reset switching device 61 is turned on while the main switching device 51 is off and the excitation current flows through the parasitic diode of the reset switching device 61, resulting in reducing a loss when the reset switching device 61 is turned on. On the other hand, the loss occurring when the main switching device 51 is turned on can be reduced, for example, by turning on the main switching device 51 when the reset switching device 61 is turned off and then the voltage across the main switching device 51 drops to zero (see FIG. 6).
However, the voltage drop time it takes for the voltage across the main switching device 51 to drop to zero from the time when the reset switching device 61 is turned off is varied with the variation of the input voltage VL and the output voltage Vo of the DC-DC converter 50 and, therefore, it is difficult to specify the time when the voltage across the main switching device 51 would drop to zero and hence the time when the main switching device 51 should be turned on. This is for example because there is no clear correlation between the voltage drop time and the input voltage VL and also between the voltage drop time and the output voltage Vo, as shown in FIGS. 7A and 7B. Thus, it is difficult to turn on the main switching device 51 when the voltage across the main switching device 51 drops to zero, which makes it difficult to reduce the loss occurring when the main switching device 51 is turned on in the DC-DC converter 50.
For example, increasing the leakage inductance of the primary coil of the transformer 54 thereby to extend the period in which the voltage across the main switching device 51 is zero makes it easy to turn on the main switching device 51 while the voltage across the main switching device 51 is zero.
However, such increased leakage inductance of the primary coil of the transformer 54 shortens the transmission period from the primary coil to the secondary coil of the transformer 54, resulting in a drop of the maximum output voltage of the DC-DC converter 50. Further, the size of the primary coil of the transformer 54 needs to be enlarged, resulting in an increased cost and an enlarged circuit size.
The present invention is directed to providing an active clamp DC-DC converter that allows reduction of loss occurring when the main switching device is turned on while preventing a drop of the maximum output voltage and an increase of cost and circuit size of the DC-DC converter.